rth from the Sun,
we have recourse to the fine planet Venus, whose orbit is situated
inside the terrestrial orbit. Owing to the combination of the Earth's
motion with that of the Star of the Morning and Evening, the capricious
Venus passes in front of the Sun at the curious intervals of 8 years,
113-1/2 years less 8 years, 8 years, 113-1/2 years plus 8 years.

Thus there was a transit in June, 1761, then another 8 years after, in
June, 1769. The next occurred 113-1/2 years less 8 years, _i.e._,
105-1/2 years after the preceding, in December, 1874; the next in
December, 1882. The next will be in June, 2004, and June, 2012. At these
eagerly anticipated epochs, astronomers watch the transit of Venus
across the Sun at two terrestrial stations as far as possible removed
from each other, marking the two points at which the planet, seen from
their respective stations, appears to be projected at the same moment on
the solar disk. This measure gives the width of an angle formed by two
lines, which starting from two diametrically opposite points of the
Earth, cross upon Venus, and form an identical angle upon the Sun. Venus
is thus at the apex of two equal triangles, the bases of which rest,
respectively, upon the Earth and on the Sun. The measurement of this
angle gives what is called the parallax of the Sun--that is, the angular
dimension at which the Earth would be seen at the distance of the Sun.

[Illustration: FIG. 83.--Measurement of the distance of the Sun.]

Thus, it has been found that the half-diameter of the Earth viewed from
the Sun measures 8.82". Now, we know that an object presenting an angle
of one degree is at a distance of 57 times its length.

The same object, if it subtends an angle of a minute, or the sixtieth
part of a degree, indicates by the measurement of its angle that it is
60 times more distant, _i.e._, 3,438 times.

Finally, an object that measures one second, or the sixtieth part of a
minute, is at a distance of 206,265 times its length.

Hence we find that the Earth is at a distance from the Sun of
206,265/8.82--that is, 23,386 times its half-diameter, that is,
149,000,000 kilometers (93,000,000 miles). This measurement again is as
precise and certain as that of the Moon.

I hope my readers will easily grasp this simple method of triangulation,
the result of which indicates to us with absolute certainty the distance
of the two great celestial torches to which we owe the radiant light of
day